I used a combination of GWL trees and AC21 trees in my model. The enclosed screenshot illustrates a bug(?) if CineRender is used in creating a fly-through.
The left side of the enclosed screen shot is a CineRender image of the model, based on one of the cameras I used for the fly-through. The right side of the image is a frame of the fly-through from the same camera. In the fly-through, the AC21 trees render correctly, but the GWL billboard trees, which render correctly when camera views are rendered one at a time, come out as solid grey slabs. The GWL simple trees come out as grey planes. The first frame of the fly-through is correct, but all the subsequent frames have the grey slabs.
I assume this has to do with some time-saving algorithm in the fly-through process, which doesn't work with the older GWL trees. But I had no inkling that the GWL trees were a problem as I developed the views and tested them individually. It was only when I put them all together as a fly-through that I became aware of the problem.
My main reason for using GWL trees rather than AC21 trees is that AC21 trees can't pick up the height and width parameters from other "tree" library parts, while the GWL trees can. Here's another case where working around one problem generated a new problem down the line....